I believe actually that the patina is a chemical reaction with the surface layer of the metal. It changes the chemical composition of the surface and makes it less susceptible to rusting because it is not the same carbon steel as it was before.

The reason we see antique blades that are in great condition is because of the patina that protects them. I also see it on a lot of instruments, good trumpets and saxophones etc, antique silverware etc, its a natural protection. If not for it then, following the some of the logic in this thread, we would need a vacuum with no oxygen at all to keep a razor from rusting